Human's brain is the most flexible and highly adaptable regarding sensitivity, perception, thinking, behavior, etc. Human's brain consists of hundreds of billions of nerve cells and each nerve cell is connected with other nerve cells to form various interactions, which is called Synapse. The interaction is the key to the human's mental activity such as study, memory, recognition, activity, decision, etc. and is responsible for a human's physical control function to maintain health. Synapse is formed by a chemical reaction and the chemical reaction is converted into a scalp's electric flow to form a brainwave. That is, hundreds of billions of nerve cells send information with an interaction with other nerve cells and during this process, an electric signal is generated. Therefore, if an electrode is put into the scalp to measure electrical changes, the electrical changes are expressed as a wave. This wave is a brainwave (electroencephalogram; EEG).
The brainwave has various shapes depending on the degree of the brain activity. The frequency of the brainwave increases as the brain activity is high and the frequency of the brainwave decreases as the brain activity is low. The brainwave is divided into a gamma wave, a beta wave, an alpha wave, a theta wave and a delta wave.
A gamma wave is the fastest wave with a frequency between 38 to 45 Hz and with a mean frequency of 40 Hz and is generated when a human is nervous or in an active and highly complex mental process.
A beta wave is a wave with a frequency between 15 to 38 Hz and with a mean frequency of 17.3 Hz and is generated in a cerebrum during an active brain process. Further, a beta wave is generated when a human is nervous or suffers from stress. In particular, a high-beta wave is generated during an excessive activity, e.g., when a human suffers from an obsession, an excessive concentration and an anxiety.
An alpha wave is a wave with a frequency between 8 to 12 Hz and with a mean frequency of 10.3 Hz and is generated in an occipital lobe in a awakened state which is not a an unconscious state or a subconscious state with eyes closed. An alpha wave is mainly generated in a stress-relaxation state and is often generated before or after a severe stress.
A theta wave is a wave with a frequency between 4 to 8 Hz and with a mean frequency of 6.3 Hz and is generated dominantly in a pleocortex which mainly relates to emotion or sensitivity. Since a theta wave mainly relates to an area of emotion or sensitivity, it is generated considerably when a human makes an artistic effort, gets hurt, is happy, is working cheerfully or is playing.
A delta wave is a wave with a frequency between 0.5 to 4 Hz and with a mean frequency of 1.3 Hz and has the largest amplitude such that it has a strong penetration force to move furthest. A delta wave is generated mainly in a myelencephalon, a pons or a mesencephalon which relates to life and is generated mainly when a human is in deep sleep which stops activities in a paleocortex area relating to emotion and a neocortex area relating to information input/output and thought.
Meanwhile, a research on a new brainwave called SMR (Sensory Motor Rhythm) between a beta wave and an alpha wave is being carried out actively. A SMR wave is a wave between 12 to 15 Hz and with a mean frequency of 12.7 Hz. A SMR wave is generated dominantly in a cerebrum (neocortex) from an ear to a brain.
A SMR wave is generated when a problem requiring a simple concentration is being solved or when a relaxation is needed in a conscious state. That is, a SMR wave is generated when a human can concentrate on an easy and simple work accurately without nervousness or stress. A SMR wave turned out to be a brainwave generated in a concentrated state in that a SMR wave allows all matters to be solved with a very small amount of energy compared to beta wave.
Using measurement values of the brainwaves as above, a human's state such as a mentality or a mental condition can be expressed with an index. In one example, as shown in Formula 1 below, it is being suggested how a concentration index, a relaxation index, a stress index, etc. can be calculated by each of the above explained waves which consist the brainwave.concentration index=(SMR wave+M beta wave)/theta waverelaxation index=alpha wave/H beta wavestress index=high beta wave  [Formula 1]
Regarding a technology using the above brainwave, a method to measure a degree of a user's stress through brainwaves and use the measurements is being suggested. In one example, Korean laid-open publication No. 10-2007-0061311 titled “Recognition of stress state by brainwave and relaxation system and method using music” discloses a technique to remove stress by acquiring a stress state more reliably using characteristic information which is obtained from the brainwave during analyzing a brainwave to understand the degree of stress and by playing music which is set depending on the acquired stress state.
However, in prior arts including the technique disclosed in Korean laid-open publication No. 10-2007-0061311, only one brainwave feature, e.g., a magnitude of a high-beta wave, is being used to measure a user's brainwave. It is disadvantageously incorrect to evaluate a user's stress index only by a brainwave since a user's stress characteristic is generated in the form of various bio-signals.
On the other hand, to evaluate a stress index based on various bio-signals, an apparatus for detecting more than two bio-signals should be mounted on a human. While a wearable device is becoming known recently, it is not easy to design an apparatus for detecting more than two bio-signals as a wearable device. Therefore, it is not easy for a user to measure a stress index accurately in a daily life and to reflect it on the life.